1. Field of the Invention
The present invention relates to a storage system and an operation method thereof, and more particularly to a storage system which can set, when backing up data on a magnetic tape, suitable search performance depending on the purpose of the backing-up, and an operation method thereof.
2. Related Art
For the backing-up of data stored in a high capacity storage apparatus such as a disk array apparatus, magnetic tape devices are widely used. In a magnetic tape device, data is written in serial from the head of a tape. Thus, unlike a disk device, it is not possible to make a direct access to a block to/from which data is written/read.
Thus, in the writing of data from a disk device, a tape drive records an identifier, which is a so-called file mark (hereinafter, referred to as an “FM”) representing tape position information, on a magnetic tape by providing a non-signal interval, for example, every time a certain amount of data is written. When stored data is read out, data stored in a designated block is extracted by positioning the reading head of the tape drive to an FM recorded immediately before the data to be read, and continues to read the data up until a succeeding FM.
In relation to a technology achieving fast data writing and reading by recording on a magnetic tape position information of such as the above FMs, Japanese Patent Application Laid-open Publication No. 4-286018 proposes a method in which a correspondence is recorded on the tape, the correspondence being that between position information of data blocks and position partition information (file marks or the like) recorded on a tape and such information is expanded and used at the time of reading so that search performance can be enhanced in search speed. Further, Japanese Patent Application Laid-open Publication No. 2008-41178 proposes a method in which position information for the shortening of search time of data recorded on a file basis is stored in a header part, and recorded in the trailing end of a tape.
As described above, while it is possible to enhance the performance of searching data on a magnetic tape by recording FMs on the magnetic tape, a processing for recording an FM on the magnetic tape and a processing for reading the recorded FM each require three to five seconds at each position. For this reason, the smaller the intervals (which are each represented by an amount of data (e.g., in units of GB) recorded between FMs, and which are hereinafter referred to as “FM intervals”) between FMs to be recorded, the closer the reading head can be positioned to a target data. On the other hand, there may be a case in which time required for recording FMs at the time of data writing is increased, and as a result performance of writing data on the tape is reduced.
In view of such tendency, in a backup processing in which much importance is placed on fast writing performance from a disk device to a magnetic tape, large FM intervals are taken in order to reduce the number of FMs to be recorded on a magnetic tape so that performance can be improved.
On the other hand, when a magnetic tape is used for the purpose of a file archive, a main processing is to read out specific information in the magnetic tape, i.e., to read out a specific file included in a logical unit stored in the magnetic tape. Thus, when FM intervals are large, there was a problem that the positioning to and reading of specific information would take time.